In fabrication of the SOI wafer by the ion implantation delamination method, when a bond wafer that forms an SOI layer (Silicon On Insulator layer, in a broad sense, Semiconductor On Insulator) and a base wafer have been bonded together via an oxide film and thereafter it has been delaminated by performing heat treatment on an ion implanted layer, an SOI layer front surface (a delaminating plane) of the SOI wafer after delamination and a front surface (a delaminating plane) of the bond wafer after delamination mutually face. Therefore, there were cases where contact took place between the wafers depending on shapes of warps of the SOI wafer after delamination and the bond wafer after delamination and scratches and SOI film thickness abnormality were generated.
Against the problem, there was a method for fabricating the SOI wafer by forming an oxide film on the entire surface of the bond wafer and bonding it with the base wafer. In this method, since while the SOI wafer warps into a convex shape by a buried oxide film transferred from a front surface of the bond wafer, the bond wafer after delamination has no oxide film on a bonded face and warps into a concave shape reversely to the SOI wafer by the oxide film remained on a back surface, and the magnitudes of warps become the same as each other between the SOI wafer and the bond wafer after delamination, occurrence of contact between the wafers can be made difficult theoretically. However, since in reality, also the influence of a warped shape when processing the wafer still remains, for example, in a case where the wafer shape when processing the bond wafer is the convex shape, the bond wafer after delamination turns to a shape that the convex shape upon wafer processing has been taken off from the concave shape caused by the influence of the back surface oxide film. At this time, there are cases where mismatching occurs between the warped shapes of the SOI wafer and the bond wafer after delamination and the magnitude of the warp of the concave shape of the bond wafer after delamination becomes small in comparison with the magnitude of the warp of the convex shape of the SOI wafer. In such a case, at a tip part of the convex shape of the SOI wafer, contact with the bond wafer after delamination took place and the scratches and the SOI film thickness abnormality were generated.
Among them, generation of the SOI film thickness abnormality is induced because formation of a natural oxide film on the front surface of the SOI wafer is obstructed by contact of the SOI wafer with the bond wafer and the natural oxide film on a contact part becomes thin. That is, in SC1 cleaning (cleaning with an NH4OH/H2O2 solution) of RCA cleaning of the next process, the oxide film is early removed from a part which is thin in oxide film of the wafer contact part and an Si front surface of an SOI layer is exposed, and etching of Si with SC1 is started relatively in early stage. As a result, after SC1 cleaning, a thin film part is formed in the film thickness of the SOI by the part which was thin in the oxide film (the wafer contact part).
In addition, as an ion implantation method for forming an ion implanted layer which would induce delamination, there is an ion implantation delamination method which is performed by implanting two kinds of, for example, hydrogen ions and helium ions, performed by so-called co-implantation. In this method, as shown in FIG. 4, when a bond wafer 101 (for example, a warped shape: 60 μm convex) that an oxide film 102 (for example, 27 nm) has been formed and a helium ion implanted layer 104 has been set at a position deeper than that of a hydrogen ion implanted layer 103 and a base wafer 105 are bonded together via the oxide film 102 so as to form a bonded wafer 110, although the hydrogen ion implanted layer 103 is divided into parts on the side of an SOI wafer 120 having an SOI layer 106 and on the side of a bond wafer 101′ after delamination in a delaminating process, the helium ion implanted layer 104 remains in the bond wafer 101′ after delamination still after delamination. In this case, due to presence of the helium ion implanted layer 104, a force warping toward the convex side works on the bond wafer 101′ after delamination and mismatching in warped shape between the bond wafer 101′ (for example, 40 μm convex) after delamination and the SOI wafer 120 (for example, 20 μm convex) occurs. Therefore, in the method for manufacturing SOI wafer by the ion implantation delamination method utilizing co-implantation, there were cases where contact with the bond wafer after delamination occurred at the convex shape tip part of the SOI wafer and the scratches and the SOI film thickness abnormality were generated.